The new field of nanotechnology is giving us a better view of what is happening at the physio-chemical level as cement hydrates and concrete cures. Nuclear resonance reaction analysis (NRRA) is being used to study cement hydration at a nanoscale level. The result is a better idea of what takes place on the surface of the cement particle as it hydrates, leading to improved industry standards and guidelines for mixing and curing concrete.

NRRA uses a beam of nitrogen atoms to probe a reacting cement grain to locate hydrogen atoms, a necessary component of water, or its reaction products. The results of the probe are plotted in a graph called a hydrogen depth profile, which shows the rate of penetration of the water. This also indicates the arrangement of the various surface layers formed during the reaction.

The 20-nanometer-thick surface layer acts as a semi-permeable barrier that allows water to enter the cement grain and calcium ions to leach out. However, the larger silicate ions in the cement are trapped behind this layer.

As the reaction continues, a silicate gel layer forms beneath the surface layer, causing swelling within the cement grain and eventually leading to breakdown of the surface layer. This breakdown releases the accumulated silicate into the surrounding solution, where it reacts with calcium ions to form a calcium-silicate hydrate gel, which binds the cement grains together and sets the concrete.

The evolution of the hydrogen profile shows the time of breakdown of the surface layer. This information can be used to study the concrete setting process as a function of time, temperature, cement chemistry, and other factors. For example, researchers used NRRA to determine that in cement hydrating at 30 deg C (86 deg. F), the breakdown occurs at 1.5 hours.

Mineral Admixtures: Fly Ash

Mineral admixtures like fly ash, silica fume or microsilica, and slags are usually added to concrete to enhance the workability of fresh concrete; to improve resistance of concrete to thermal cracking, alkali-aggregate expansion, and sulfate attack; and to enable a reduction in cement content or water.

An industrial byproduct, fly ash is the residuum of the combustion of coal for electricity generation; its minute particles are finer than those of cement and are glassy-spherical in shape.

Fly ash is a pozzolan, meaning it is a siliceous and aluminous material that, in the presence of water, will combine with an activator (lime, portland cement or kiln dust) to produce a cementitious material, according to Fly Ash Facts for Highway Engineers, a publication of the FHWA, and authored by the American Coal Ash Association (ACAA).

The American Society for Testing and Materials (ASTM) classifies fly ash into Class C and Class F categories. Class C, with a higher calcium oxide content (CaO, or “lime”), comes from the burning of western sub-bituminous or lignite coals, and Class F is derived from bituminous eastern coals. Fly ash with high amounts of residual carbon is not suitable for use with concrete.

Research indicates that Class F fly ash is better suited for fighting alkali-silica reactivity (ASR), the common malady of concrete. As ASR’s presence is a function of deleterious aggregates, we will address it in our July installment, The Chemistry of Aggregates.

As it chemically reacts with cement, fly ash benefits concrete in these seven ways:

• Gains in concrete strength. Mix designs containing fly ash that produce equivalent strength at early ages – fewer than 90 days – will ultimately exceed the strength of conventional PCC mixes, according to Fly Ash Facts for Highway Engineers.

• Improved workability. This is due to the spherical shape and minute size of fly ash particles. The round shape lends flowability, actually lubricating the mix, which can speed construction of placement of concrete in forms for structural elements, both in the field and at the precast plant.

• Reduced bleeding of mix water. Lower amounts of water are required due to the mix’s improved workability.

• Reduced heat of hydration. Fly ash reacts more slowly than straight portland cement when substituted for portland cement. This benefits mass pours such as for bridge footings, as a slower set equals fewer cracks.